6.2 Horizon in 86 Convincor Need Help with Poor Performance

[Mark A]

I had a motor and engine replaced last year. It has been a nightmare. The shop messed up the job real bad. I ended up taking it to another shop to re do all the work that was done. Now, a year later and a lot of money dropped I need to prop the boat and I am confused, I am hoping someone will have suggestions.

The boat with a 1986 350 Mag (260 hp) and 1.47 alpha drive did 49 mph with a 19' aluminum prop at about 4900 rpm. I don't recall having a major issue with porpoising with that set up.

With the new motor & drive (2005 6.2 Horizon MPI 320 HP and 1999 Bravo 1 with 1.5 gear set) I tried a 21" 3 blade aluminum prop but do not get much past 4000 rpm. When I attempt trim up I get severe porpoising. I also tried a 19" mirage plus stainless prop and saw 4500 and close to 50 mph but with even slight trim up introduced severe porpoising.

I understood, going from alpha to bravo should not change the x dimension.

This 6.2 motor WOT is 5000 - 5200.

I believe the weight between the old package and new is pretty close.

The 1986 package is rated for 260 at the crank with the 2005 6.2 engine rated at 320 HP at the prop. Is this correct?

So how can the performance be poorer with this package?

It is almost August and I am thinking about putting a match to it!!! Please help!

 

[JW]

The first thing I'm wondering is how many inches your propshaft centerline is below your pad, when you have the outdrive trimmed to level. Should be around 4.5".
Next try more props. Porpoising usually means not enough power or not enough prop or both. Doesn't the Horizon come with a computer that can be read by a hand held computer? I'm wondering if you're building good power at WOT. Might have an issue with that motor.

 

[vinny p.]

"The 1986 package is rated for 260 at the crank with the 2005 6.2 engine rated at 320 HP at the prop. Is this correct?"


Yes that is correct. You can expect a Bravo to eat up 30 hp. So, your new engine is 90 hp more than the old one.

Your problems can be a host of things. First and foremost is rigging. Are you certain that it is rigged correctly? Fuel system, wiring??
When referring to your "X" dimension. A common misconception is that the "X" is a measurement of prop shaft height. That is incorrect. The "X" is the distance between the bottom of the hull and the crankshaft centerline. The distance that the centerline of the prop shaft is below the hull is the prop shaft height. With that said, I am almost certain your "X" has changed. I am not sure if your prop shaft height has changed. Only a tape measure will tell you for sure. I never did this type of upgrade from an Alpha to Bravo. Doesnt the cut out have to be changed to allow for the bigger gimbal? Doesnt that require the engine to be in a bit higher? While I never measured them side by side, I thought that a Bravo is taller than an Alpha. That alone can change the prop shaft height big time.

The symtoms you described make me think the propshaft is too low. How far below the bottom is it? Did you measure the old set up before doing the upgrades? If not , maybe a call to the factory can tell you what it was originally set to.

 

[Mark A]

Hi Vinny, Thanks for your input. The cut out was not changed. When I had reserched it I believe I found that the dimesions were the same. Speaking to various installers I got a lot of deiiferent opionions on this but I know the cut out was not changed. Driving the boat it seems like either the motor does not have enough power or the prop is too high as as soon as I try to trim out the porpoising gets more severe and I am never seeing the rpms I should be. I do not have the full smartcraft package so I am worried the "engine guardian" could be limiting the power on the engine. I am not able to get clear information from merc on this but from what I have read diferent alsrms will alter the output poer to protect it anywhere from 10 to 90 %. I think I would need the merc tester hooked in while driving the boat or the smartcraft monitor package to know if engine guardian is limiting the power.

I will make measurments asap. I did not measure the alpha set up so I will need to count on the factory for this information. It gets complicated I think too as if the weight changed, which I believe I added 200 or so lbs, the stern would sit deeper. If the stern sits deeper would this alter where you want the prop height? Has any one swapped an alpha for a bravo on these boats? I would think this would be fairly common.

 

[Mark A]

HI JW, Not enough power and not enough prop or prop height. I am hoping it is some simple motor issue or that the horizon engine guardian is limiting the power for some simple alarm. I need to get the smartcraft tester or someone who has it to verify what if any alarms are going on. I have only the smartcraft tach and it is not showing any engine alarms though. If the prop height needs to change up or dowm it will be very difficult. There simply is not a lot of clearance below (stringers) or above this engine and it would require everything to come out again. Maybe I shoulld post a new thread on swapping alpha for bravo drive. I have heard a lot of conflicting information concerning doing this. When I looked at it myself it seemed to me that they both had the same length with the bravo being slightly heavier. Maybe I should swap the bravo for an Alpha . . .

 

[vinny p.]

The engine may be going into guardian mode. You will need a scanner to tell for sure. Propshaft height is easily lowered by adding a spacer. They are available in 1/2" increments up to 3" . Assuming your drive currently doesnt have a spacer, raising it will be a problem. You will have to get a shorter lower section, thats the easiest way.

 

[bigsbetter]

I don't have any input on the power issue,but maybe some on the porpoising. My 251 Convincor porpoises just as you stated yours does now. It's a characteristic of a convincor I guess. I'm not sure if the hulls are anything alike, but when I was doing some testing runs and I had my top engine hatch/cover off and my back seats out, the boat did not porpoise at all! The combined weight removed from the back was around 250-300lbs. After I reinstalled the hatch and seats ,now the porpoising returns. I'm now working on moving my batteries all the way to a compartment in the front of the bow,and also rearranging all my storage so I get as much weight as possible up front to see if the porpoising will vanish again. Just thinking if the weight addition in your boat could have an effect on your porpoising also.

 

[Mark A]

I will be going out om Monday with a marine mechanic who is bringing the scanner so we should be able to check for codes. I will need to look into the spacers. Are they easy to install? How expensive are they?

 

[Mark A]

This is very interesting. I think the weight increased by 200 or so lbs. While the engine was out I cleaned and painted the bilge and moved the batteries back to the stern from where they were just behind the seat. This opened up the engine compartment and gave it a cleaner look but I shifted a chunk of weight back quite a bit I doing this. I also had removed the refrigerator making the bow a little lighter. Let me know how you make out moving the batteries forward. People I mentioned this to thought the battery weight was not enough to matter. I agree, but won't rule it out. I read someplace that you can experiment with placing weight up in the bow, like maybe a sand bag or 2 to get an idea of the effect of changing the balance of the boat. Maybe I will try that too. If you are moving the batterys to the bow you will need really heavy guage cables to carry the DC voltage to start the engine.

 

[bigsbetter]

Yes the cables are going to have to big extra big. I'm thinking of using some cables that are used in high end car audio. They make huge cables in 00,000,and0000 sizes. The big ones are about the diameter of a half dollar coin. I'm wondering how much weight they will add??? Like you said, Maybe I'll expierement with some sand bags before I go through this.

 

[vinny p.]

Spacers run about $550 each. The problem is that it will be a guess as to which one to try first. Its an easy job to do. You just have to split the 2 halves of the drive, remove the studs and put everything back together. In case you dont know, the new coupler goes taper end up when putting everything back together.

 

[Mark A]

Some Testingplanned for Monday

Thanks again Vinny,

My marine mechanic is bringing his tester tommorow and we will run the boat. I will make a series of measurements including the prop shaft height. I am also going to look for a hook or rocker along the bottom of the boat. From all that I read though, hooks and rockers are often best left alone as the manufactuer often intentially puts them in to improve the performance.

Also, as far as prop shaft height, what is more critical, the relationship of the prop centerline to the boat bottom, or the prop centerline to the surface of the water when the boat is at rest in the water? Is that a stupid question? It is something that has confused me because if the boat is heavier due to weight which will vary with load it will make the boat and prop sit lower in the water. Does this not matter as once on plane the prop will end up near the surface as the hull comes up in the water?

I think I am going to find a combination of issues here.

1. This brand new engine is having power limited by the engine guardian system.

2. The change in weight all though small combined with a slight change in prop shaft height has increased the porpoising effect.

3. Because the engine guardian is kicked in, I can not make enough power to turn the prop and make proper rpm.

 

[vinny p.]

Mark,

It doesnt matter how deep the propshaft sits in the water when the boat is at rest. Have you ever noticed a 29' Fountain with twin big blocks? That thing looks like its going to sink at the dock. All you are concerned with is how far the shaft is below the bottom of the boat. You are right, unless you really know what you are doing, dont mess around with the bottom. Manufacturers work hard at finding a point where everything is happy and safe. Sure, you can get more speed but at the expense of handling.

 

[Mark A]

Update

My marine techs came down to the boat, hooked up the merc tester and found no alarms other then one for the horn circuit. They were not able to troubleshoot that but all else checked out fine.

We went for a ride and the engine sounds real good but does not seem to be able to turn this prop as it should and we have a lot of porpoising as well. He is going to talk to Merc engine rep for ideas.

I also made measurements of the prop shaft height. I found with the drive level, from the bottom of the boat to the center line of the prop the distance was 5.5 inches. This would seem to be right in the ball park of where it should be. From reading a doc from the merc web site on mercury propellers, the tip of the prop blade for work boats can be 1 - 2 " below the bottom of the boat, normal use the tip of the blade would be in line with the bottom of the boat, ski & sport 1 - 3 inches above and high performance 4" or more above. Based on a 14.25" prop my prop tip measures 1.625" above the boat bottom when trimmed to 0. This is in line with ski and sport.

So I am waiting for my marine mechanic to come up with some more ideas as he feels the boat should be doing better and I have another potential issue to check. When the boat was at the original marina who hacked up the engine replacement it was left outside and not covered. It was during some intense rain and I know it was allowed to fill with water for a period of time.

I am wondering if I have a lot of water trapped under the floor boards. Not sure the best way to check. I had replaced the fuel tank and floor boards which were soft when I picked up the boat a few years ago and I feel they were sealed up real well with epoxy resin but if the foam floatation is filled with water perhaps this extra weight is causing the poor performance.

 

[daren34]

weight

go to a public scale and weigh it

make shure that the tow vehicle and the trailer are on different sections of the scale

for exact weight unhook the trailer and reweight the tow vehicle

daren

oh almost all truckstops have scales